Wind turbine generator, active damping method thereof, and windmill tower

RELATED APPLICATIONS

0001The present application is a continuation of U.S. application Ser. No. 13/220,121, filed Aug. 29, 2011, which is a continuation of U.S. application Ser. No. 12/727,356, filed Mar. 19, 2010 (now U.S. Pat. No. 8,026,623), which is a divisional of U.S. application Ser. No. 10/590,328, filed Jun. 25, 2007 (now U.S. Pat. No. 7,692,322), which is a U.S. National Stage of PCT/JP2004/16851, filed Nov. 12, 2004, and claims priority from Japanese Application Number 2004-055515, filed Feb. 27, 2004, the disclosures of which are hereby incorporated by reference herein in their entirety.

TECHNICAL FIELD

0002The present invention relates to wind turbine generators, active damping methods thereof, and windmill towers in which vibrations induced by fluctuations of wind speed can be suppressed. In particular, the present invention relates to wind turbine generators, active damping methods thereof, and windmill towers in which vibrations of the wind turbine generators themselves or the windmill towers can be reduced at low cost and without increasing the weight of a nacelle.

BACKGROUND ART

0003Wind turbine generators generally have a structure in which heavy objects such as blades, a gearbox, and a generator are provided at the top of a cylindrical tower having a height of several tens of meters; therefore, vibrations induced by fluctuations of wind speed are extremely large. Such vibrations increase the fatigue loading of structural components, resulting in a decrease in the life of the windmill tower.

0004Recently, the size of wind turbine generators has been increased. As the size of the generators increases, the effect of vibrations induced by fluctuations of wind speed becomes more significant. Thus, reducing the vibrations in wind turbine generators or windmill towers is a critical technical problem.

0005On the other hand, in tall structures such as high-rise buildings, active damping techniques are used in order to improve the living conditions during strong winds. Various methods have been proposed, but most of them involve driving a heavy object (mass) provided on the upper part of the structure with an actuator such as a motor to absorb the vibrations of the structure itself; one example is an active mass damper (AMD).

0006However, when the active damping technique (AMD) used in high-rise structures and the like is directly applied to wind turbine generators or windmill powers without modification, the following problems occur.

0007First, in order to achieve a satisfactory damping effect, a considerably heavy object (mass) is necessary. Furthermore, in order to drive this considerably heavy object, an actuator having a large capacity must be provided. Consequently, the weight of the nacelle significantly increases.

0008Secondly, since the weight of the nacelle disposed at the top of a windmill tower increases, the strength of the windmill tower supporting the nacelle must be increased accordingly. This need to significantly increase the strength of the windmill tower and other components increases the total cost of the wind turbine generator and the windmill tower.

0009Thirdly, an actuator for driving the heavy object (mass) is necessary. Accordingly, the number of parts for driving is increased, resulting in increased maintenance costs.

0010To solve the above problems, for example, Japanese Unexamined Patent Application Publication No. 2001-221145 (Patent Document 1) discloses a technique in which vibrations of a windmill tower are suppressed by providing a passive-active pitch-flap mechanism. <ul id="ul0001" list-style="none"><li id="ul0001-0001" num="0011">[Patent Document 1]</li><li id="ul0001-0002" num="0012">Japanese Unexamined Patent Application Publication No. 2001-221145</li></ul>

DISCLOSURE OF INVENTION

0013However, the invention described in Patent Document 1 ultimately employs a method of reducing vibrations of the windmill tower using a mechanical mechanism. Therefore, this method is no different from the known AMD method, resulting in an increase in the weight of the nacelle. Furthermore, a plurality of structures are included, resulting in problems such as an increase in the size of the nacelle and an increase in the cost.

0014The present invention has been made in order to solve the above problems, and an object of the present invention is to provide wind turbine generators, active damping methods thereof, and windmill towers in which vibrations can be reduced at low cost and without increasing the weight of the nacelle.

0015In order to solve the above problems, the present invention provides the following solutions.

0016The present invention provides a wind turbine generator including a pitch-angle control mechanism for controlling a pitch angle of windmill blades on the basis of a blade-pitch-angle command, wherein the wind turbine generator includes an accelerometer, attached to a nacelle, for detecting the acceleration due to vibrations of the nacelle; and an active damping unit for calculating a pitch angle of the windmill blades for generating a thrust on the windmill blades so as to cancel out the vibrations of the nacelle on the basis of the acceleration detected with the accelerometer and for outputting a blade-pitch-angle command to the pitch-angle control mechanism.

0017According to the present invention, the acceleration due to vibrations of the nacelle is detected with the accelerometer attached to the nacelle, a pitch angle of the windmill blades for generating a thrust on the windmill blades so as to cancel out the vibrations of the nacelle is calculated in the active damping unit on the basis of the acceleration, and the pitch angle is output as a blade-pitch-angle command to the pitch-angle control mechanism, thereby controlling the pitch angle of the windmill blades. In this case, the drag acting on the windmill blade acts as a thrust in the front-rear direction of the nacelle, and the magnitude of the thrust varies depending on wind speed and the pitch angle of the windmill blade. Accordingly, when the pitch angle is controlled on the basis of a predetermined control rule, vibrations in the front-rear direction of the nacelle can be controlled to some extent.

0018The present invention also provides a wind turbine generator including a pitch-angle control mechanism for controlling a pitch angle of windmill blades on the basis of a blade-pitch-angle command, wherein the wind turbine generator includes an accelerometer, attached to a nacelle, for detecting the acceleration due to vibrations of the nacelle; an active damping unit for calculating a pitch angle of the windmill blades for generating a thrust on the windmill blades so as to cancel out the vibrations of the nacelle on the basis of the acceleration detected with the accelerometer and for outputting a blade-pitch-angle command for damping; a pitch-angle control unit for calculating a pitch angle of the windmill blades for controlling the output of the wind turbine generator to be a predetermined value on the basis of wind speed, the rotational speed of a windmill rotor, or the output of the wind turbine generator and for outputting a blade-pitch-angle command for output control; and an adder for supplying the pitch-angle control mechanism with a blade-pitch-angle command obtained by combining the blade-pitch-angle command for damping output from the active damping unit with the blade-pitch-angle command for output control output from the pitch-angle control unit.

0019According to the present invention, the acceleration due to vibrations of the nacelle is detected with the accelerometer attached to the nacelle. A pitch angle of the windmill blades for generating a thrust on the windmill blades so as to cancel out the vibrations of the nacelle is calculated in the active damping unit on the basis of the acceleration, and the pitch angle is output as a blade-pitch-angle command for damping. On the other hand, a pitch angle of the windmill blades for controlling the output to be a predetermined value is calculated in the pitch-angle control unit, and the pitch angle is output as a blade-pitch-angle command for output control. The blade-pitch-angle command for damping is combined with the blade-pitch-angle command for output control by the adder. Thus, the pitch angle of the windmill blades is controlled on the basis of the resulting blade-pitch-angle command after combining.

0020Since the technique of pitch-angle control has been widely employed to date for the purpose of output control, the present invention can be realized by merely additionally mounting the accelerometer, the active damping unit, and the adder on an existing wind turbine generator. Accordingly, the cost of installing and operating the active damping control can be markedly reduced, and thus vibrations of the wind turbine generator can be reduced at low cost. Furthermore, since the pitch-angle control is performed by combining the blade-pitch-angle command for damping with the blade-pitch-angle command for output control, output control and damping control can be achieved at the same time.

0021In the wind turbine generator of the present invention, the active damping unit preferably includes a speed estimation unit for estimating a speed from the acceleration detected with the accelerometer, and a control unit for calculating a pitch angle of the windmill blades for generating a thrust on the windmill blades so as to cancel out the vibrations of the nacelle on the basis of the speed output from the speed estimation unit.

0022According to this invention, in the active damping unit, the speed estimation unit estimates a speed from the acceleration detected with the accelerometer. The control unit then calculates a pitch angle of the windmill blades for generating a thrust on the windmill blades so as to cancel out the vibrations of the nacelle on the basis of the estimated speed.

0023Since the active damping unit can be realized by a simple structure including the speed estimation unit and the control unit, vibrations of the wind turbine generator can be reduced at low cost.

0024In the wind turbine generator of the present invention, the speed estimation unit preferably integrates the acceleration detected with the accelerometer to calculate the speed.

0025Since the speed estimation unit integrates the acceleration detected with the accelerometer to calculate the speed, noise in the high-frequency band can be removed. Thereby, the control unit in the subsequent stage can perform stable and effective damping control.

0026In the wind turbine generator of the present invention, the control unit preferably includes a phase-lead compensator for advancing the phase of the speed output from the speed estimation unit by a predetermined amount, and preferably calculates the pitch angle on the basis of the speed obtained after the phase-lead compensation.

0027Furthermore, the control unit preferably includes a phase-lag compensator for delaying the phase of the speed output from the phase-lead compensator by a predetermined amount, and preferably calculates the pitch angle on the basis of the speed obtained after the phase-lag compensation.

0028According to this invention, the pitch angle is calculated on the basis of the speed obtained after the phase-lag compensation. Since the phase-lag of the output of the accelerometer can be compensated for and noise in the high-frequency band can be reduced, stable and effective damping control can be performed.

0029In the wind turbine generator of the present invention, the control unit preferably includes any one of a proportional controller, a proportional-integral controller, a proportional-integral-derivative controller, a linear-quadratic regulator, and a linear-quadratic Gaussian regulator to which the speed estimated by the speed estimation unit is input to calculate the pitch angle.

0030When the control unit has such a structure, stable and effective damping control can be performed.

0031In the wind turbine generator of the present invention, the active damping unit preferably includes a limiter for limiting the pitch angle of the windmill blades or the angular speed of the pitch angle of the windmill blades to a predetermined range.

0032According to this invention, the active damping unit, more specifically, the control unit provided in the active damping unit, includes a limiter for limiting the pitch angle of the windmill blades or the angular speed (rate of change) of the pitch angle of the windmill blades to a predetermined range. Therefore, fatigue of the pitch-angle control mechanism can be reduced, and problems due to errors in setting the parameters or the like can be prevented.

0033Furthermore, when the blade-pitch-angle command for damping is limited to a much smaller range than the blade-pitch-angle command for output control, effects caused by interference of both command values can be decreased or prevented.

0034The present invention provides an active damping method of a wind turbine generator including a pitch-angle control mechanism for controlling a pitch angle of windmill blades on the basis of a blade-pitch-angle command, and an accelerometer, attached to a nacelle, for detecting the acceleration due to vibrations of the nacelle, the active damping method including an active damping step of calculating a pitch angle of the windmill blades for generating a thrust on the windmill blades so as to cancel out the vibrations of the nacelle on the basis of the acceleration detected with the accelerometer and outputting a blade-pitch-angle command to the pitch-angle control mechanism.

0035According to the present invention, the accelerometer attached to the nacelle detects the acceleration due to vibrations of the nacelle, a pitch angle of the windmill blades for generating a thrust on the windmill blades so as to cancel out the vibrations of the nacelle is calculated in the active damping step on the basis of the acceleration, and the pitch angle is output as a blade-pitch-angle command to the pitch-angle control mechanism, thereby controlling the pitch angle of the windmill blades. Thus, the control can be realized by the accelerometer, hardware of the pitch-angle control mechanism, and software of the active damping step. Therefore, the cost of installing and operating the active damping control can be markedly reduced, and vibrations of the wind turbine generator can be reduced at low cost.

0036The present invention provides an active damping method of a wind turbine generator including a pitch-angle control mechanism for controlling a pitch angle of windmill blades on the basis of a blade-pitch-angle command, and an accelerometer, attached to a nacelle, for detecting the acceleration due to vibrations of the nacelle, the active damping method including an active damping step of calculating a pitch angle of the windmill blades for generating a thrust on the windmill blades so as to cancel out the vibrations of the nacelle on the basis of the acceleration detected with the accelerometer and outputting a blade-pitch-angle command for damping; a pitch-angle control step of calculating a pitch angle of the windmill blades for controlling the output of the wind turbine generator to be a predetermined value on the basis of wind speed, the rotational speed of a windmill rotor, or the output of the wind turbine generator and outputting a blade-pitch-angle command for output control; and an addition step of supplying the pitch-angle control mechanism with a blade-pitch-angle command obtained by combining the blade-pitch-angle command for damping in the active damping step with the blade-pitch-angle command for output control in the pitch-angle control step.

0037According to this invention, an accelerometer attached to the nacelle detects the acceleration due to vibrations of the nacelle, and a pitch angle of the windmill blades for generating a thrust on the windmill blades so as to cancel out the vibrations of the nacelle is calculated in the active damping step on the basis of the acceleration to output the pitch angle as a blade-pitch-angle command for damping. On the other hand, a pitch angle of the windmill blades for controlling the output to be a predetermined value is calculated in the pitch-angle control step to output a blade-pitch-angle command for output control. The blade-pitch-angle command for damping is combined with the blade-pitch-angle command for output control in the addition step, and the pitch angle of the windmill blades is controlled on the basis of the resulting blade-pitch-angle command after combining. Since the technique of pitch-angle control has been widely employed to date for the purpose of output control, the present invention can be realized merely adding the active damping step and the addition step to existing control software on a wind turbine generator.

0038Thus, since the control can be realized by mounting the accelerometer and adding the software, the cost of installing and operating the active damping control can be markedly reduced, and vibrations of the wind turbine generator can be reduced at low cost. Furthermore, since the pitch-angle control is performed by combining the blade-pitch-angle command for damping with the blade-pitch-angle command for output control, output control and damping control can be achieved at the same time.

0039In the active damping method of a wind turbine generator of the present invention, the active damping step preferably includes a speed estimation step of estimating a speed from the acceleration detected with the accelerometer, and a control step of calculating a pitch angle of the windmill blades for generating a thrust on the windmill blades so as to cancel out the vibrations of the nacelle on the basis of the speed estimated in the speed estimation step.

0040According to this invention, in the active damping step, a speed is determined in the speed estimation step on the basis of the acceleration detected with the accelerometer, and a pitch angle of the windmill blades for generating a thrust on the windmill blades so as to cancel out the vibrations of the nacelle is calculated in the control step on the basis of the speed. Since the active damping step can be realized by a simple structure including the speed estimation step and the control step, vibrations of the wind turbine generator can be reduced at low cost.

0041In the active damping method of a wind turbine generator of the present invention, the speed estimation step preferably integrates the acceleration detected with the accelerometer to calculate the speed.

0042Since noise in the high-frequency band can be removed by the speed estimation step, stable and effective damping control can be performed in the active damping step.

0043In the active damping method of a wind turbine generator of the present invention, the control step preferably includes a phase-lead compensation step of advancing the phase of the speed output from the speed estimation step by a predetermined amount and calculates the pitch angle on the basis of the speed obtained after the phase-lead compensation.

0044In the active damping method of a wind turbine generator of the present invention, the control step preferably includes a phase-lag compensation step of delaying the phase of the speed output from the phase-lead compensation step by a predetermined amount and calculates the pitch angle on the basis of the speed obtained after the phase-lag compensation.

0045According to this invention, the control step includes a phase-lead compensation step of advancing the phase of the speed output from the speed estimation step by a predetermined amount, the control step includes a phase-lag compensation step of delaying the phase of the speed output from the phase-lead compensation step by a predetermined amount, and the pitch angle is calculated on the basis of the speed obtained after the phase-lag compensation. Accordingly, since the phase-lag of the output of the accelerometer can be compensated for and noise in the high-frequency band can be reduced, stable and effective damping control can be performed.

0046In the active damping method of a wind turbine generator of the present invention, the control step preferably includes a compensation step of performing any one of a proportional control, a proportional-integral control, a proportional-integral-derivative control, a control using a linear-quadratic regulator, and a control using a linear-quadratic Gaussian regulator for the speed estimated by the speed estimation step and calculates the pitch angle on the basis of the speed obtained after the compensation.

0047Thereby, stable and effective damping control can be performed.

0048In the active damping method of a wind turbine generator of the present invention, the active damping step preferably includes a limiting step of limiting the pitch angle of the windmill blades or the angular speed of the pitch angle of the windmill blades to a predetermined range.

0049According to this invention, fatigue of the pitch-angle control mechanism can be reduced, and problems due to errors in setting the parameters or the like can be prevented. Furthermore, when the blade-pitch-angle command for damping is limited to a much smaller range than the blade-pitch-angle command for output control, effects caused by interference of both command values can be decreased or prevented.

0050The wind turbine generator of the present invention can be suitably used for a windmill tower.

0051By applying the wind turbine generator of the present invention to a windmill tower, the cost of installing and operating the active damping control can be markedly reduced, and vibrations of the windmill tower can be reduced at low cost. Furthermore, unlike the known AMD method, since a heavy object (mass) and an actuator for the heavy object are not used, the weight of the nacelle does not increase and the strength of the windmill tower need not be increased. Thus, vibrations of the windmill tower can be reduced at low cost.

0052According to the wind turbine generator of the present invention, vibrations can be suppressed by an accelerometer, an active damping unit, and a pitch-angle control mechanism without using a heavy object and an actuator for driving the heavy object, which are used in the known AMD method. Consequently, the cost of installing and operating the active damping control system can be markedly reduced, resulting in an advantage that vibrations of the wind turbine generator can be reduced at low cost.

BRIEF DESCRIPTION OF DRAWINGS

0053<figref idref="DRAWINGS">FIG. 1</figref> is a diagram showing the configuration of a wind turbine generator according to an embodiment of the present invention.

0054<figref idref="DRAWINGS">FIG. 2</figref> is a view illustrating forces acting on a windmill blade.

0055<figref idref="DRAWINGS">FIG. 3</figref> is a graph showing an example of the relationship between the thrust and the pitch angle for a change in wind speed.

0056<figref idref="DRAWINGS">FIG. 4(</figref><i>a</i>) is a schematic view of a windmill tower, and <figref idref="DRAWINGS">FIG. 4(</figref><i>b</i>) is a schematic diagram in the case where the windmill tower is modeled as a mechanical vibration system.

0057<figref idref="DRAWINGS">FIG. 5</figref> is a block diagram of an active-damping control system according to an embodiment of the present invention.

0058<figref idref="DRAWINGS">FIG. 6</figref> includes block diagrams each showing an example of the structure of a control unit of an active damping unit.

0059<figref idref="DRAWINGS">FIG. 7</figref> is a flow chart showing an example of the details of the control of a limiter shown in <figref idref="DRAWINGS">FIG. 6</figref>.

0060<figref idref="DRAWINGS">FIG. 8</figref> is a flow chart showing an example of the details of the control of the limiter shown in <figref idref="DRAWINGS">FIG. 6</figref>.

0061<figref idref="DRAWINGS">FIG. 9</figref> is a block diagram of a control system in the case where the active-damping control system is installed in an output control system.

0062<figref idref="DRAWINGS">FIG. 10</figref> is a graph showing the characteristic of output from a windmill generator versus wind speed.

0063<figref idref="DRAWINGS">FIG. 11</figref> is a graph showing an example of the frequency characteristic of the vibration amplitude in a tower system in cases where the active damping is performed and is not performed by the active damping unit.

BEST MODE FOR CARRYING OUT THE INVENTION

0064Embodiments of a wind turbine generator, an active damping method thereof, and a windmill tower of the present invention will now be described in detail with reference to the attached drawings.

0065<figref idref="DRAWINGS">FIG. 1</figref> is a diagram showing the configuration of a wind turbine generator according to an embodiment of the present invention. In the figure, the wind turbine generator of this embodiment includes a mechanical part <b>10</b> of the wind turbine generator, an active damping unit <b>20</b>, a pitch-angle control unit <b>30</b>, and a subtracter <b>40</b>. First, the outline of the components in the wind turbine generator of this embodiment will be described.

0066The mechanical part <b>10</b> of the wind turbine generator includes a windmill rotor <b>11</b>, windmill blades <b>12</b>, a nacelle <b>13</b>, and an anemometer <b>16</b> as main components. The nacelle <b>13</b> includes a gearbox <b>14</b>, a generator <b>15</b>, and an accelerometer <b>17</b>.

0067In the mechanical part <b>10</b> of the wind turbine generator, a plurality of windmill blades <b>12</b> attached to the windmill rotor <b>11</b> receive wind power energy and are rotated together with the windmill rotor <b>11</b>. The speed is increased by the gearbox <b>14</b>, and the generator <b>15</b> is then driven to generate electricity. Thus, the wind power energy is converted to electrical energy. In <figref idref="DRAWINGS">FIG. 1</figref>, the structure includes the gearbox <b>14</b>, but a direct drive system that does not include the gearbox <b>14</b> may also be used.

0068The accelerometer <b>17</b>, which is a feature of the wind turbine generator of this embodiment, is disposed inside the nacelle <b>13</b> and at a position close to the central part of the tower. The accelerometer <b>17</b> detects the acceleration due to vibrations in the front-rear direction of the nacelle <b>13</b>.

0069The pitch-angle control unit <b>30</b> calculates a pitch angle of the windmill blades <b>12</b> for controlling an output P of this wind turbine generator to be a predetermined value on the basis of a wind speed v measured with the anemometer <b>16</b>, a rotational speed N of the windmill rotor <b>11</b>, or the output P of the wind turbine generator, and outputs the pitch angle as a blade-pitch-angle command θ* for output control. This output control by controlling the pitch angle has been performed in the known art, and the pitch-angle control unit <b>30</b> of this embodiment is the same as that of the known art.

0070The active damping unit <b>20</b> calculates a pitch angle of the windmill blades <b>12</b> for generating a thrust on the windmill blades <b>12</b> so as to cancel out vibrations of the nacelle <b>13</b> on the basis of the acceleration detected with the accelerometer <b>17</b>, and outputs the pitch angle as a blade-pitch-angle command δθ* for damping.

0071The subtracter (adder) <b>40</b> combines the blade-pitch-angle command δθ* for damping obtained from the active damping unit <b>20</b> with the blade-pitch-angle command θ* for output control obtained from the pitch-angle control unit <b>30</b> and provides the result to the pitch-angle control mechanism as a blade-pitch-angle command. Here, the pitch-angle control mechanism (not shown in the figure) controls the pitch angle of the windmill blades <b>12</b> on the basis of the blade-pitch-angle command, and the structure thereof and the like are the same as those of the known art.

0072Next, the detailed structure of the active damping unit <b>20</b> and an active damping method for reducing vibrations of the wind turbine generator and the windmill tower using the active damping unit <b>20</b> will be described in detail.

0073First, the basic principles of the active damping method will now be described with reference to <figref idref="DRAWINGS">FIGS. 2 and 3</figref>. <figref idref="DRAWINGS">FIG. 2</figref> shows a cross-section of one of the windmill blades <b>12</b> (see <figref idref="DRAWINGS">FIG. 1</figref>) viewed from the leading end of the windmill blade <b>12</b> toward the base thereof, and illustrates forces acting on the windmill blade <b>12</b>. In the figure, the rotation direction of the windmill blade is the direction from the right to the left, and the vibration direction of the wind turbine generator or the windmill tower is the vertical (x) direction. <figref idref="DRAWINGS">FIG. 3</figref> is a graph showing an example of the relationship between the thrust and the pitch angle in the case where the wind speed v is varied from 6 to 24 [m/s].

0074As shown in <figref idref="DRAWINGS">FIG. 2</figref>, during the windmill operation, a lift L and a drag D act on the windmill blade. The drag D acts as a thrust in the front-rear direction of the nacelle <b>13</b> (see <figref idref="DRAWINGS">FIG. 1</figref>) of the windmill tower. As shown in <figref idref="DRAWINGS">FIG. 3</figref>, the magnitude of the thrust varies depending on the wind speed and the pitch angle. Accordingly, when the pitch angle is controlled on the basis of a control rule, by changing the thrust in the front-rear direction of the nacelle <b>13</b> of the windmill tower, vibrations in the front-rear direction of the nacelle <b>13</b> of the windmill tower can be controlled to some extent. The present invention focuses on this point, and the control rule of the pitch angle will be described below.

0075<figref idref="DRAWINGS">FIG. 4(</figref><i>a</i>) is a schematic view of the windmill tower, and <figref idref="DRAWINGS">FIG. 4(</figref><i>b</i>) is a schematic diagram in the case where the windmill tower is modeled as a mechanical vibration system. Specifically, <figref idref="DRAWINGS">FIG. 4(</figref><i>a</i>) schematically shows that the accelerometer <b>17</b> is provided in the nacelle <b>13</b> of the windmill tower to detect acceleration (d²x/dt²) for a displacement x. As shown in <figref idref="DRAWINGS">FIG. 4(</figref><i>b</i>), the windmill tower can be modeled using an object with a mass m, a spring with a stiffness k, and a dashpot with a viscous resistance c.

0076In this mechanical vibration system, when the displacement shifted from the equilibrium state is defined as x, the equation for the vibrations of the object is represented by equation (1): <br /><i>m{umlaut over (x)}+c{dot over (x)}+kx=f+Δf</i>  (1)

0077In the equation, f+Δf represents the force acting on the object, and Δf represents an additional force exerted by the pitch control operation of the active damping unit <b>20</b>. Equation (1) is transformed into equation (2):

0078<maths id="MATH-US-00001" num="00001"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mover><mi>x</mi><mi>¨</mi></mover><mo>+</mo><mrow><mfrac><mi>c</mi><mi>m</mi></mfrac><mo>⁢</mo><mover><mi>x</mi><mo>.</mo></mover></mrow><mo>+</mo><mrow><mfrac><mi>k</mi><mi>m</mi></mfrac><mo>⁢</mo><mi>x</mi></mrow></mrow><mo>=</mo><mrow><mfrac><mn>1</mn><mi>m</mi></mfrac><mo>⁢</mo><mrow><mo>(</mo><mrow><mi>f</mi><mo>+</mo><mrow><mi>Δ</mi><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mi>f</mi></mrow></mrow><mo>)</mo></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths><img file="US8395273B2_D0001.tif" />

0079Here, a natural frequency ωn of the system and a damping factor ζ are represented as follows, and thus equation (2) can be rewritten as equation (5). <br />ω<i>n</i>=(<i>k/m</i>)^1/2  (3)<br />ζ=<i>c/</i>2(<i>mk</i>)^1/2  (4)

0080<maths id="MATH-US-00002" num="00002"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><mover><mi>x</mi><mi>¨</mi></mover><mo>+</mo><mrow><mn>2</mn><mo>⁢</mo><msub><mi>ζω</mi><mi>n</mi></msub><mo>⁢</mo><mover><mi>x</mi><mo>.</mo></mover></mrow><mo>+</mo><mrow><msubsup><mi>ω</mi><mi>n</mi><mn>2</mn></msubsup><mo>⁢</mo><mi>x</mi></mrow></mrow><mo>=</mo><mrow><mfrac><mn>1</mn><mi>m</mi></mfrac><mo>⁢</mo><mrow><mo>(</mo><mrow><mi>f</mi><mo>+</mo><mrow><mi>Δ</mi><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mi>f</mi></mrow></mrow><mo>)</mo></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>5</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths><img file="US8395273B2_D0002.tif" />

0081Furthermore, equation (5) is subjected to a Laplace transformation to derive equation (6): <br /><i>s</i>²<i>X</i>(<i>s</i>)+2ζω<i>nsX</i>(<i>s</i>)+ω<i>n</i>²<i>X</i>(<i>s</i>)=(1<i>/m</i>)<i>F</i>(<i>s</i>)  (6)<br /> From equation (6), a transfer function G(s) of the system is represented by equation (7): <br /><i>G</i>(<i>s</i>)=<i>X</i>(<i>s</i>)/<i>F</i>(<i>s</i>)=(1<i>/m</i>)/(<i>s</i>²+2<i>ζωns+ωn</i>²)  (7)

0082In the second-order frequency response characteristics as in equation (7), referring to equations (3) and (4), the natural frequency ωn of the system can be changed by changing the mass m and the stiffness k; however, regarding the damping factor ζ, the effect of a change in the viscous resistance c is larger than the effect of changes in the mass m and the stiffness k.

0083On the other hand, in equation (1), the additional force Δf is set, for example, as follows. <br />Δ<i>f=−Dp</i>^{{dot over (x)}}  (8)

0084In this case, equation (1) can be rewritten as equation (9). <br /><i>m{umlaut over (x)}+</i>(<i>C+Dp</i>)<i>{dot over (x)}+kx=f</i>  (9)

0085That is, by setting the additional force Δf exerted by the pitch control operation of the active damping unit <b>20</b> as represented by equation (8), the first-order term of equation (9) is increased by +Dp, and thus the damping factor ζ can be changed to a larger value. Consequently, damping of the vibrations can be performed more rapidly, and in the frequency response characteristics, a peak value of the gain of the natural frequency ωn can be suppressed more to suppress the vibration amplitude.

0086Next, the specific structure and operation of the active damping control will be described in detail on the basis of the above-described basic principles of the active damping method. <figref idref="DRAWINGS">FIG. 5</figref> is a block diagram of an active-damping control system according to this embodiment.

0087In <figref idref="DRAWINGS">FIG. 5</figref>, reference numeral <b>51</b> indicates a pitch actuator that drives the windmill blades <b>12</b> on the basis of the blade-pitch-angle command output from the subtracter <b>40</b> to control the pitch angle. The pitch actuator <b>51</b> is specifically realized by a hydraulic cylinder, an electric motor, or the like. Here, from the standpoint of the mechanical vibration system, the pitch actuator <b>51</b> is modeled by a first-order lag system.

0088Reference numeral <b>52</b> indicates a blade system that calculates the thrust acting on the windmill blades during the windmill operation. As shown in <figref idref="DRAWINGS">FIG. 2</figref>, since the thrust in the front-rear direction of the nacelle <b>13</b> of the windmill tower is the sum of the front-rear directional components of the lift L and the drag D, an adder <b>54</b> adds these components and outputs the result. Regarding the thrust due to the drag D, the pitch angle of the windmill blade <b>12</b> and the thrust have characteristics shown in <figref idref="DRAWINGS">FIG. 3</figref>. Accordingly, the thrust is considered to be in inverse proportion to the pitch angle, and is determined with an amplifier <b>53</b> having a gain of Kb based on a gradient obtained by linear approximation of the above relationship.

0089Reference numeral <b>55</b> indicates a tower system in which the windmill tower is modeled as a mechanical vibration system. The transfer function is determined by equation (7), but in the active-damping control system, the acceleration (d²x/dt²) is detected with the accelerometer <b>17</b> and the result is fed back. Therefore, the modeling is performed using a transfer function obtained by multiplying equation (7) by s². This model is a model of only a first-order vibration mode.

0090A known wind turbine generator also has a structure including the above-described pitch actuator <b>51</b>, the blade system <b>52</b>, and the tower system <b>55</b>. In this embodiment, the accelerometer <b>17</b>, the active damping unit <b>20</b>, and the subtracter <b>40</b> are added to these components to form a feedback loop. The accelerometer <b>17</b> detects acceleration, which is the output of the tower system <b>55</b>. The active damping unit <b>20</b> generates the blade-pitch-angle command δθ* for damping used for changing the thrust in the front-rear direction of the nacelle <b>13</b> of the windmill tower. The subtracter <b>40</b> performs a calculation of δθ*−θ* so as to combine the blade-pitch-angle command δθ* for damping obtained from the active damping unit <b>20</b> with the blade-pitch-angle command θ* for output control output from the pitch-angle control unit <b>30</b>.

0091The accelerometer <b>17</b> is modeled by a first-order lag system because the output thereof includes a phase lag. In the active damping unit <b>20</b>, as set in equation (8), a value obtained by multiplying the speed (dx/dt) by Dp is defined as the additional force exerted by the pitch control operation of the active damping unit <b>20</b>. Therefore, the active damping unit <b>20</b> includes an integrator <b>21</b> that integrates the acceleration to determine the speed, and a control unit <b>22</b> having a transfer function Gc(s).

0092Specifically, the acceleration (first-order vibration mode) in the front-rear direction of the nacelle <b>13</b> is measured with the accelerometer <b>17</b> provided inside the nacelle <b>13</b>, the measured acceleration is input to the active damping unit <b>20</b>, and the speed in the front-rear direction of the nacelle <b>13</b> is calculated by an integration operation by the integrator <b>21</b>. In the control unit <b>22</b> of the active damping unit <b>20</b>, the blade-pitch-angle command δθ* for damping used for obtaining the damping effect is calculated on the basis of the calculated speed. The blade-pitch-angle command δθ* for damping determined in the active damping unit <b>20</b> is combined, by the subtracter <b>40</b>, with the blade-pitch-angle command θ* for output control determined in the pitch-angle control unit <b>30</b> (see <figref idref="DRAWINGS">FIG. 1</figref>). The pitch actuator <b>51</b> drives the windmill blades <b>12</b> on the basis of the combined blade-pitch-angle command to control the pitch angle. This pitch-angle control controls the output of the wind turbine generator. In addition, a thrust according to the pitch angle acts so as to suppress vibrations in the front-rear direction of the nacelle <b>13</b> of the windmill tower. Thus, the thrust allows the vibrations to be rapidly damped.

0093Thus, in this embodiment, by combining the blade-pitch-angle command δθ* for damping with the blade-pitch-angle command θ* for output control, output control and damping control can be achieved at the same time. The integrator <b>21</b>, which calculates the speed, not only performs the integration operation but also has a frequency characteristic that relatively suppresses a high-frequency band and emphasizes a low-frequency band. Accordingly, the integrator <b>21</b> also has a function of cutting noise in the high-frequency band.

0094The structure of the integrator is not limited to the complete integral (1/s). Alternatively, the integrator may be a filter (for example, a first-order lag element) having the same function as that of the above, an appropriate state estimator (a full-order or minimal-order observer, or a Kalman filter), or the like.

0095Next, the specific structure and operation of the control unit <b>22</b> of the active damping unit <b>20</b> will now be described with reference to <figref idref="DRAWINGS">FIGS. 6(</figref><i>a</i>) and <b>6</b>(<i>b</i>). <figref idref="DRAWINGS">FIGS. 6(</figref><i>a</i>) and <b>6</b>(<i>b</i>) are block diagrams each showing an example of the structure of the control unit <b>22</b> of the active damping unit <b>20</b>.

0096In <figref idref="DRAWINGS">FIG. 6(</figref><i>a</i>), a control unit <b>22</b><i>a </i>includes a phase-lead compensator <b>62</b>, a phase-lag compensator <b>63</b>, as amplifier <b>64</b>, and a limiter <b>65</b>.

0097As described above, since the output of the accelerometer <b>17</b> includes a phase lag, the phase-lead compensator <b>62</b> adjusts the phase. As shown in the figure, the phase-lead compensator <b>62</b> has a transfer function of a phase-lead system represented by (1+sαT<b>1</b>)/(1+sT<b>1</b>) (wherein α&lt;1).

0098When the output passes through the phase-lead compensator <b>62</b>, noise in the high-frequency band is amplified. Therefore, the phase-lag compensator <b>63</b> is added as a countermeasure, thereby relatively suppressing the high-frequency band and emphasizing the low-frequency band. As shown in the figure, the phase-lag compensator <b>63</b> has a transfer function of a phase-lag system represented by (1+sαT<b>2</b>)/(1+sT<b>2</b>) (wherein α>1). Thus, the control unit <b>22</b> of the active damping unit <b>20</b> includes two types of filter, i.e., the phase-lead compensator <b>62</b> and the phase-lag compensator <b>63</b>, thereby compensating for the phase-lag of the output of the accelerometer <b>17</b> and reducing noise in the high-frequency band. Therefore, stable and effective damping control can be performed.

0099In addition, according to equation (8), the amplifier <b>64</b> is configured to have a transfer function of a gain Dp. In this case, the gain Dp is preferably set on the basis of the result of a simulation, an experiment, or the like.

0100The structure of the control unit <b>22</b> (see <figref idref="DRAWINGS">FIG. 5</figref>) is not limited to the above-described phase compensators. Alternatively, the control unit <b>22</b> can be realized using, for example, a proportional controller, a proportional-integral controller, a proportional-integral-derivative controller, a linear-quadratic regulator (LQ regulator), or a linear-quadratic Gaussian regulator (LQG regulator).

0101When the pitch-angle control by the blade-pitch-angle command δθ* for damping is performed too frequently, the pitch-angle control mechanism is excessively moved, resulting in fatigue. Therefore, a limit (for example, ±1 [deg]) is preferably provided for the blade-pitch-angle command δθ* for damping by the limiter <b>65</b> (see <figref idref="DRAWINGS">FIGS. 6(</figref><i>a</i>) and <b>6</b>(<i>b</i>)), thereby reducing fatigue of the pitch-angle control mechanism.

0102Specifically, when the output (hereinafter referred to as “pitch-angle command”) of the amplifier <b>64</b> shown in <figref idref="DRAWINGS">FIG. 6</figref> is smaller than a predetermined minimum pitch-angle (“YES” in step SA<b>1</b> in <figref idref="DRAWINGS">FIG. 7</figref>), the minimum pitch-angle or a predetermined pitch-angle larger than the minimum pitch-angle is output as the final blade-pitch-angle command δθ* for damping (step SA<b>2</b> in <figref idref="DRAWINGS">FIG. 7</figref>). On the other hand, when the pitch-angle command is equal to or larger than the minimum pitch-angle (“NO” in step SA<b>1</b> in <figref idref="DRAWINGS">FIG. 7</figref>), it is determined whether or not the pitch-angle command is larger than a predetermined maximum pitch-angle (step SA<b>3</b> in <figref idref="DRAWINGS">FIG. 7</figref>).

0103As a result, when the pitch-angle command is larger than the maximum pitch-angle (“YES” in step SA<b>3</b> in <figref idref="DRAWINGS">FIG. 7</figref>), the maximum pitch-angle or a predetermined pitch-angle smaller than the maximum pitch-angle is output as the final blade-pitch-angle command δθ* for damping (step SA<b>4</b> in <figref idref="DRAWINGS">FIG. 7</figref>). On the other hand, when the pitch-angle command is equal to or smaller than the maximum pitch-angle (“NO” in step SA<b>3</b> in <figref idref="DRAWINGS">FIG. 7</figref>), the pitch-angle is output as the final blade-pitch-angle command δθ* for damping (step SA<b>5</b> in <figref idref="DRAWINGS">FIG. 7</figref>).

0104As described above, instead of limiting the output of the amplifier <b>64</b> itself (see <figref idref="DRAWINGS">FIGS. 6(</figref><i>a</i>) and <b>6</b>(<i>b</i>)), the rate of change of this output, that is, the angular speed of the pitch angle, may be limited to a certain range (for example, ±0.6 [deg/sec]).

0105Specifically, as shown in <figref idref="DRAWINGS">FIG. 8</figref>, a rate of change is first calculated on the basis of the previous value (hereinafter referred to as “previous value of pitch-angle command”) and a current value (hereinafter referred to as “current value of pitch-angle command”) of the output of the amplifier <b>64</b> (see <figref idref="DRAWINGS">FIG. 6</figref>) (step SB<b>1</b>). Subsequently, it is determined whether or not the rate of change is smaller than a predetermined minimum rate of change (step SB<b>2</b>). As a result, when the rate of change is smaller than the predetermined minimum rate of change (“YES” in step SB<b>2</b>), a value calculated by adding the minimum rate of change to the previous value of pitch-angle command is output as the final blade-pitch-angle command δθ* for damping (step SB<b>3</b>).

0106On the other hand, when the rate of change is equal to or larger than the minimum rate of change (“NO” in step SB<b>2</b>), it is determined whether or not the rate of change is larger than a predetermined maximum rate of change (step SB<b>4</b>). As a result, when the rate of change is larger than the maximum rate of change (“YES” in step SB<b>4</b>), a value calculated by adding the maximum rate of change to the previous value of pitch-angle command is output as the final blade-pitch-angle command δθ* for damping (step SB<b>5</b>). On the other hand, when the rate of change is equal to or smaller than the maximum rate of change (“NO” in step SB<b>4</b>), the current value of pitch-angle command is output as the final blade-pitch-angle command δθ* for damping (step SB<b>6</b>).

0107As described above, limiting the blade-pitch-angle command δθ* for damping or the rate of change of the pitch-angle command δθ* can prevent a problem where, for example, vibrations of the windmill tower are instead increased because of errors in setting the parameters of the vibration control system or the like.

0108Furthermore, since the blade-pitch-angle command δθ* for damping is limited to a much smaller range than the blade-pitch-angle command θ* for output control, effects caused by interference of both command values can be decreased or prevented.

0109In a control unit <b>22</b><i>b </i>shown in <figref idref="DRAWINGS">FIG. 6(</figref><i>b</i>), a second-order oscillatory compensator <b>61</b> is added to the previous stage of the phase-lead compensator <b>62</b> of the control unit <b>22</b><i>a </i>to realize more precise control.

0110In the above description, the active damping unit <b>20</b> is composed of hardware and outputs the blade-pitch-angle command δθ* for damping. Alternatively, each component may be composed of a subprogram that is sequentially executed. In this case, the integrator <b>20</b> is replaced with an integration step (speed estimation step) and the control unit <b>22</b> is replaced with a control step. Components in the control unit <b>22</b> are also replaced with a phase-lead compensation step, a phase-lag compensation step, a limiting step, and the like. These steps forms a subprogram executed in a central processing unit (CPU), a micro processing unit (MPU), or a digital signal processor (DSP) in a controller.

0111Next, the output control using the pitch-angle control unit <b>30</b> (see <figref idref="DRAWINGS">FIG. 1</figref>) will be briefly described with reference to <figref idref="DRAWINGS">FIG. 9</figref>, which shows a block diagram of a control system in the case where the above-described active-damping control system using the active damping unit <b>20</b> is installed in an output control system, realized in a known wind turbine generator, that uses the pitch-angle control unit <b>30</b>.

0112In <figref idref="DRAWINGS">FIG. 9</figref>, the pitch-angle control unit includes subtracters <b>31</b> and <b>32</b>, a wind-speed control unit <b>33</b>, a rotational-speed control unit <b>34</b>, an output control unit <b>35</b>, and a selecting unit <b>36</b>.

0113The wind-speed control unit <b>33</b> sets a blade-pitch-angle command θ_von the basis of a wind speed v [m/s] measured with the anemometer <b>16</b> and outputs the command. The rotational-speed control unit <b>34</b> sets a blade-pitch-angle command θ_Nso as to provide a predetermined rotational speed (target value) N* on the basis of a rotational speed N [rpm] of the windmill rotor <b>11</b> and outputs the command. Furthermore, the output control unit <b>35</b> sets a blade-pitch-angle command θ_Pso as to provide a predetermined output (target value) P* on the basis of an output P [kW] of the wind turbine generator and outputs the command.

0114In the selecting unit <b>36</b>, among the blade-pitch-angle commands θ_v, θ_N, and θ_Pdetermined in the wind-speed control unit <b>33</b>, the rotational-speed control unit <b>34</b>, and the output control unit <b>35</b>, respectively, the minimum value is selected (minimum selection), that is, a blade-pitch-angle command that produces the lowest output is selected, and is output as a blade-pitch-angle command θ* for output control. In general, characteristics between the output P [kW] of the windmill generator and wind speed v [m/s] are illustrated as shown in <figref idref="DRAWINGS">FIG. 10</figref>. Control is performed on the basis of the wind speed v [m/s] until a rated output and a rated wind speed are achieved. After reaching the rated output and the rated wind speed, control is performed on the basis of the rotational speed N [rpm] of the windmill rotor <b>11</b> or the output P [kW] of the wind turbine generator.

0115The control range of the pitch angle by the pitch-angle control unit <b>30</b> is large and ranges from a fine pitch (which is about −20 [deg] and at which the rotational speed is high) to a feathering pitch (which is about −104 [deg] and at which the rotational speed is low).

0116Next, the advantage of the wind turbine generator and the active damping method thereof according to this embodiment will be described with reference to example results of a simulation experiment. <figref idref="DRAWINGS">FIG. 11</figref> shows a frequency characteristic of the vibration amplitude in the tower system <b>55</b> (see <figref idref="DRAWINGS">FIG. 5</figref>) in cases where the active damping is performed and is not performed by the active damping unit <b>20</b> (see <figref idref="DRAWINGS">FIG. 1</figref>). The figure shows that the vibration amplitude is substantially suppressed near the natural frequency of the tower system <b>55</b>. Since the natural frequency of the tower system <b>55</b> is known in advance, more appropriate vibration control can be realized by setting parameters of the control system in accordance with the natural frequency.

0117As described above, in the wind turbine generator or the active damping method thereof according to this embodiment, as shown in <figref idref="DRAWINGS">FIG. 1</figref>, the acceleration due to vibrations of the nacelle <b>13</b> is detected with the accelerometer <b>17</b> attached to the nacelle <b>13</b>, a pitch angle of the windmill blades <b>12</b> for generating a thrust on the windmill blades <b>12</b> so as to cancel out the vibrations of the nacelle <b>13</b> is calculated in the active damping unit <b>20</b> (active damping step) on the basis of the acceleration, and the pitch angle is output as a blade-pitch-angle command δθ* for damping. On the other hand, a pitch angle of the windmill blades <b>12</b> for controlling the output to be a predetermined value is calculated in the pitch-angle control unit <b>30</b> (pitch-angle control step), and the pitch angle is output as a blade-pitch-angle command θ* for output control. Subsequently, the blade-pitch-angle command δθ* for damping is combined with the blade-pitch-angle command θ* for output control using the subtracter <b>40</b> (addition step), and the pitch angle of the windmill blades is controlled on the basis of the resulting blade-pitch-angle command after combining.

0118Since the technique of pitch-angle control has been widely employed to date for the purpose of output control, this embodiment can be realized by merely additionally mounting the accelerometer <b>17</b>, the active damping unit <b>20</b> (active damping step), and the subtracter <b>40</b> (addition step) on an existing wind turbine generator. Since the mounting can be easily performed, the cost of installing and operating the active damping control can be markedly reduced, and thus vibrations of the wind turbine generator can be reduced at low cost. Furthermore, since the pitch-angle control is performed by combining the blade-pitch-angle command δθ* for damping with the blade-pitch-angle command θ* for output control, output control and damping control can be achieved at the same time.

0119In the wind turbine generator of this embodiment or the active damping method thereof, as shown in <figref idref="DRAWINGS">FIG. 1</figref>, in the active damping unit <b>20</b> (active damping step), the acceleration detected by the accelerometer is integrated with the integrator <b>21</b> (integration step) to determine the speed, and a pitch angle of the windmill blades for generating a thrust on the windmill blades so as to cancel out vibrations of the nacelle is calculated by the control unit <b>22</b> (control step) on the basis of the speed. Thus, according to the present invention, since the active damping unit <b>20</b> (active damping step) can be realized using a simple structure, i.e., the integrator <b>21</b> (integration step) and the control unit <b>22</b> (control step), vibrations of the wind turbine generator can be reduced at low cost. Furthermore, since noise in the high-frequency band can be removed through the integrator <b>21</b> (integration step), stable and effective damping control can be performed.

0120According to the wind turbine generator of this embodiment or the active damping method thereof, as shown in <figref idref="DRAWINGS">FIGS. 1</figref>, <b>6</b>(<i>a</i>), and <b>6</b>(<i>b</i>), the control unit <b>22</b> (control step) includes the phase-lead compensator <b>62</b> (phase-lead compensation step) that advances the phase of the speed output from the integrator <b>21</b> (integration step) by a predetermined amount and the phase-lag compensator <b>63</b> (phase-lag compensation step) that delays the phase of the speed output from the phase-lead compensator <b>62</b> (phase-lead compensation step) by a predetermined amount, and calculates a pitch angle on the basis of the speed obtained after the phase-lag compensation. Thereby, the phase-lag of the output of the accelerometer can be compensated for and noise in the high-frequency band can be reduced, and thus stable and effective damping control can be performed.

0121According to the wind turbine generator of this embodiment or the active damping method thereof, as shown in <figref idref="DRAWINGS">FIGS. 6(</figref><i>a</i>) and <b>6</b>(<i>b</i>), the control unit <b>22</b> (control step) includes the limiter <b>65</b> (limiting step) that limits the calculated pitch angle to a predetermined range. Therefore, fatigue of the pitch-angle control mechanism can be reduced, and problems due to errors in setting the parameters or the like can be prevented. Furthermore, when the blade-pitch-angle command δθ* for damping is limited to a much smaller range than the blade-pitch-angle command θ* for output control, effects caused by interference of both command values can be reduced or prevented.

0122The embodiments of the present invention have been described in detail with reference to the drawings. However, the specific structures are not limited to the embodiments, and also include design changes that do not depart from the essence of the present invention.

0123In the above description of the embodiments, the wind turbine generator and the active damping method thereof have been described in detail. The wind turbine generator of the embodiments and the active damping method thereof can be directly applied to a windmill tower without modification. In this case, in addition to the above-described advantages, the following advantages are also provided. Namely, unlike the known AMD method, since a heavy object (mass) and an actuator for the heavy object are not used, the weight of the nacelle <b>13</b> does not increase and the strength of the windmill tower itself need not be increased. Thus, the vibrations of the windmill tower can be reduced at low cost.

0124In the embodiments, the output control is performed by the pitch-angle control. However, the present invention can also be applied to a wind turbine generator or a windmill tower that employs other output controls. In this case, however, a pitch-angle control mechanism that controls the pitch angle of the windmill blades <b>12</b> must be added.

0125Furthermore, in the actual operation, from the standpoint of increasing reliability and safety, the following structure or the method can also be employed.

0126In an example of the method, two accelerometers constantly operate inside the nacelle <b>13</b> for fail-safe operation, and only the detection result obtained from one of the accelerometers is used for the active damping control. If either of the accelerometers breaks down, the active damping control is automatically stopped.

0127When set values of parameters (mainly a feedback gain Gc(s)) of the damping control system are not appropriate, for example, when the sign is inverted or a high gain exceeding the tolerance limit is set, the damping control system becomes unstable, resulting in an increase in vibrations of the windmill tower (nacelle <b>13</b>). In an example of the method, such a state is automatically detected (with the accelerometer <b>17</b> or the like) to automatically stop the active damping control.